To group all invertebrates together is an immodest proposal, since the definition of "invertebrate" is any animal without a spinal column — no less than 97 percent of all animal species on Earth. Invertebrates range from spiders and scorpions to centipedes and millipedes, crustaceans, insects, horseshoe crabs, worms, leeches, earthworms, marine bristle worms, mussels and clams, snails, squid and octopi, sea anemones and corals, among others. The vast diversity encompassed by the term invertebrates says less about the species than it does about our typical, very unscientific habit of giving the term equal footing with the much more narrowly representative “birds” or “mammals.”

It is not known precisely how many invertebrate species exist on Earth or how many of them are at risk of extinction, though the International Union for the Conservation of Nature's Red List puts the global total at about 1.3 million species and the percentage at risk of extinction at about 30 percent of species evaluated.

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